#include<stdio.h>

int w0, w, m;
void dichotomy(double a, double b)
{
	int i;
	double A = w0, r = (a + b) / 2.0;
	if ((b - a) < 0.00001)
	{
		printf("%.1lf", r * 100);
		exit(0);
	}
	for (i = 0; i < m; i++)
	{
		A = A * (1 + r) - w;
	}
	if (A < 0)
		dichotomy(r, b);
	if (A > 0)
		dichotomy(a, r);
	if (A == 0)
	{
		printf("%.1lf", r);
		exit(0);
	}
}
int main_4()
{
	scanf("%d %d %d", &w0, &w, &m);
	dichotomy(0, 3);
	return 0;
}